Retroreflective materials are employed for various safety purposes. Particularly, these materials are useful at night time when visibility is critical under low light conditions. This is especially important for firemen's coats and protective clothing where visibility is important. However, the conditions that firemen are exposed to are exceedingly harsh, especially in regard to excessive heat and temperature conditions. Many retroreflective materials are made of plastics that soften at temperatures of about 212.degree. F. (100.degree. C.). The softened plastic in such materials begins to flow causing the material to lose its retroreflectivity.
The clothing and any trim on the clothing for firemen must meet standards established by the National Fire Protection Association (NFPA). The NFPA has recently proposed a new and stricter standard, entitled "Standard on Protective Clothing for Structure Fire Fighting No. 531." This proposed standard requires that the outerlayer material of the fire fighting protective coating pass a set of laboratory test conditions which include flame resistance, heat resistance, fluorescence, photometry, and seam strength. In the test for heat resistance, test materials are placed in a forced convection oven at a temperature of 500.degree. F. (260.degree. C.) for a period of at least five minutes. During the test, the materials must not melt, separate, or ignite. In the test for flame resistance, the test material is exposed to a direct flame of a bunsen burner for twelve seconds. During this time, the test material must char less than four inches while not dripping or melting during exposure to the flame. Further, once the flame is turned off, the test material must have an afterflame of less than two seconds. A structure that meets these standards with respect to flame retardance, which is defined as the resistance to catching fire when exposed to a direct flame, and heat resistance, which is defined as the resistance to melting, separating, or igniting when exposed to substantial heat, is hereinafter considered a fire-resistant structure.
This proposed standard further requires that the firemen's coats be trimmed with 325 square inches of a retroreflective tape which meets the standard. The tape is applied in the form of bands around the sleeves and on the bottom of the coat.
One type of retroreflective material is formed of cube-corner or prism retroreflectors, such reflectors, are described in U.S. Pat. No. 3,712,706, issued to Stamm (Jan. 23, 1973). Generally, the prisms are made by forming a master die on a flat surface of a metal plate or other suitable material. To form the cube corners, three series of parallel equidistance intersecting v-shaped grooves at 60.degree. angles to each other are inscribed in the flat plate. The die is then used to form the desired cube-corner array into a flat plastic surface. When the groove angle is 70 degrees, 31 minutes, 43.6 seconds, the angle formed by the intersection of two cube faces (the dihedral angle) is 90 degrees, and the incident light is retroreflected back to the source.
The efficiency of a retroreflective structure is a measure of the amount of incident light returned within a cone diverging from the axis of retroreflection. Distortion of the prism structure adversely affects the efficiency. Furthermore, cube-corner retroreflective elements have low angularity, i.e., the elements will only brightly reflect light that impinges on it within a narrow angular range centering approximately on its axis of retroreflection. Low angularity arises from the inherent nature of these elements which are trihedral structures having three mutually perpendicular lateral faces. The elements are arranged so that the light to be retroreflected impinges into the internal space defined by the faces, and retroreflection of the impinging light occurs by total internal reflection of the light from face to face of the element. Impinging light that is inclined substantially away from the axis of retroreflection of the element (which is the trisector of the internal space defined by the faces of the element) strikes the face at an angle less than its critical angle, thereby passing through the face rather than being reflected.
Further details concerning the structures and the operation of cube-corner microprisms can be found in U.S. Pat. No. 3,684,348, issued to Rowland (Aug. 15, 1972), incorporated in its entirety by reference herein. A method for making retroreflective sheeting is also disclosed in U.S. Pat. No. 3,689,346, issued to Rowland (Sep. 5, 1972), also incorporated by reference herein. The method disclosed in U.S. Pat. No. 3,689,346, teaches forming cube-corner microprisms in a cooperatively configured mold. The prisms are bonded to sheeting to form a composite structure in which the cube-corner formations project from one surface of the sheeting.